Special guis?

Hello,

I have a question. If you press F3 and F4 in the latest versions of Minecraft at the same time, such an inventory opens or a gui you can program yourself or somehow call that e.g. No need to use inventories anymore because I find this variant much better.

Sure you can. For this you need a mod (modification), since this mod is already very special, you will not get around programming yourself.

Most use the Forge Modloader and therefore also the ForgeAPI.

Tutorials abound on YT, have fun.

I rather thought that I built in a plugin city inventory

What do you mean exactly? For a server? With key combination etc? Please explain exactly.

So I would like to open a gui with a command etc. So far I've only ever used inventories, now I would like to use such a menu (on a spigot server in a plugin)

Unfortunately that does not work. This requires a MOD. But if you try hard, a deeply nested and well-designed inv can be just as good.